1. Field of the Invention
The present invention relates to a seawater circulating system for improving breeding conditions in fisheries, and more particularly to a seawater circulating system for improving the breeding conditions in fisheries by driving a pump provided underwater to dissipate and eliminate oxygen-deficient water masses, thereby holding the dissolved oxygen concentration in the seawater to at least a predetermined value.
2. Description of Related Art
The confinement of marine areas is rising sharply due to the concentration of human populations in urban areas and modernization in river and harbor facilities, as a result of which the development of the surface-fed fisheries industry inside bays has led to an increased influx and accumulation of biomass to the confined marine areas, thus causing so-called artificial eutrophication. The self-cleansing ability of these marine areas has been comparatively reduced with the reconstruction and modernization of rivers, seashores and bays, so that marine areas which have a tendency to allow pollution of the water and sediment are becoming more common. A typical pollution pattern based on eutrophication of confined marine areas is one where the seawater stratifies in times of high water temperature and the bottom layer stagnates, causing large amounts of oxygen to be consumed in the decomposition of the accumulated organic material, thereby making the water of the bottom layer deficient in oxygen or devoid of oxygen and forming stagnant oxygen-deficient water masses. When this continues for a long period of time, the sea floor becomes anaerobic (i.e. becomes an oxygen-less environment), as a consequence of which the bottom mud begins to perform circulatory decomposition, generating sulfides, causing blackening and releasing the stench of hydrogen sulfide.
Confined marine areas which have reached this state can give rise to mass deaths of fish and other marine life and be a source of foul smells. Thus, they are highly likely to sustain severe blows both as natural marine environments and as fisheries.
The cycle of oxygen use in fishery installations inside bays is such that oxygen is consumed by the oxidative decomposition of wastes, feed residues and the like of the fish being bred and respiratory bacteria among the plankton, while the oxygen is replenished by the influx of fresh seawater from outside, photosynthesis of phytoplankton and dissolution from the air.
Since fresh water enters bays more often as a result of the action of the tides rather than of ocean currents, the consumption of oxygen exceeds the supply at times of high water temperature during which the oxygen dissolution rate is low, thus causing oxygen-deficient conditions. That is, oxygen-deficient or oxygen-less conditions are caused by the reduction of the oxygen supply and rise of bottom layer oxygen-deficient water masses.
On the other hand, while fisheries can be largely divided into open-sea and in-bay types, most fisheries operate within the confined marine areas of bays. However, since the circulation of seawater in these types of marine areas more often depends on the tides than on ocean currents, oxygen-deficient conditions can often arise in these fisheries in the summer and into autumn, depending on the location, thus forming oxygen-deficient water masses. There are reports that the amount of dissolved oxygen inside bays is generally lower than on the open ocean. Oxygen-deficient water masses can be a direct cause of suffocation leading to death in fish, which can lead to disastrous consequences. Additionally, whereas the risks involved in aquaculture in confined areas include oxygen-deficient water masses, proliferation of parasites and infection by pathogens, the proliferation of parasites and contamination by pathogens can be avoided to a certain extent if sufficient oxygen is provided by preventing the formation of oxygen-deficient water masses, thereby reducing the risks to fisheries.
In view of the above circumstances, the present invention has the purpose of improving and maintaining the water quality inside bays where fish are cultivated, and achieving a habitat that is amenable for fish and other marine life. The inventors of the present invention have noted the fact that the supply of seawater from the open ocean into bays is controlled more by the tides than by the currents, and the fact that while oxygen-deficient marine areas occur near the ocean floor, the oxygen content in the seawater near the surface is held comparatively high by the photosynthesis of phytoplankton and dissolution from the atmosphere, thus coming to the conclusion that oxygen-deficient and oxygen-less areas can be easily dissipated and eliminated by stirring the sea floor, especially the water of the bottom layer, by means of a seawater pump.
Thus, in a first aspect of the present invention, the inventors propose a system comprising a seawater pump provided underwater and an oxygen concentration sensor provided near the sea floor, the system being capable of eliminating oxygen-deficient areas by operating the seawater pump and stirring the water of the bottom layer when the measured oxygen concentration goes below a predetermined value.
Additionally, the inventors have noted that the natural agitation of water inside bays is controlled more by the tides than by the currents, and have thereby discovered that it is effective to stir the seawater in a vertical direction when supplementing natural agitation using a seawater pump.
Thus, in a second aspect of the present invention, the inventors propose a system comprising a seawater pump provided underwater and an oxygen concentration sensor provided near the sea floor, the system being capable of eliminating oxygen-deficient areas by operating the seawater pump and stirring the water of the bottom layer while simultaneously introducing seawater with a high dissolved oxygen concentration from the surface layer to the bottom layer when the measured oxygen concentration goes below a predetermined value.
Furthermore, the inventors observed that the conditions under which oxygen-deficient marine areas occur, being decided by such factors as the tides, currents, temperature changes and sunlight, fluctuate in a roughly one-year cycle, thus discovering that it is possible to predict to when and in which areas oxygen-deficient conditions will occur.
According to a third aspect of the present invention, the inventors propose a method for operating a system comprising a seawater pump provided underwater, the system operating method being such that the occurrence of oxygen-deficient areas is prevented by operating the seawater pump beforehand at times when the occurrence of oxygen-deficient areas are predicted based on annual oxygen-deficient area occurrence data.
Finally, the inventors have noted that one of the reasons for the comparatively high dissolved oxygen concentration near the ocean surface is the photosynthesis of phytoplankton near the surface, thus discovering that it is possible to induce photosynthesis in phytoplankton in the water of the bottom layer by introducing sunlight thereto.
Thus, according to a fourth aspect of the present invention, the inventors propose a sunlight collecting apparatus comprising a light receiving portion capable of being provided on the water surface, a light scattering portion capable of being provided near the sea floor and a light conducting portion for conducting light from the light receiving portion to the light scattering portion.
Other features and structures of the present invention aside from those mentioned above should be made clear from the description of the embodiments with reference to the attached drawings.